1. Field of the Invention
The present application relates to a driving circuit, in particular, the application relates to a driving circuit for transmitting an optical signal with a plurality of logic levels applicable to the Pulse Amplitude Modulation (PAM) system.
2. Background Arts
In optical transmission systems of core communication networks and local communication systems between servers in data centers, optical transceivers, which is are commonly used to convert electrical signals to optical signals and vice versa, transmit the optical signals with other optical transceivers in high speed.
Generally, such an optical transceiver comprises a transmission part (optical transmitter) and a reception part (optical receiver). The transmission part comprises a semiconductor laser as a light source, an optical modulator, and a driving circuit in order to convert an electrical signal to an optical signal and transmit the optical signal into an optical fiber cable. The optical modulator modulates a CW (Continuous Wave) light provided from the light source by a driving signal. The driving circuit provides the driving signal and also a bias level as a reference of the driving signal. Such a modulation system using a light source and an optical modulator is called as the external modulation system. Specifically, some optical transceivers use Electro-absorption Modulated Laser (EML), in which an Electro-Absorption Modulator (EAM) and a semiconductor laser are integrated on the same semiconductor chip, to meet the requirements of the miniaturization and the low power consumption for downsizing the optical transmission system and increasing the communicating channels into the optical transmission systems in higher density.
Optical transceivers have some specific external forms, called Form Factor, for instance, XFP, QSPP+, CFP, and so on. Each of the form factor is individually defined by MSA (Multi-Source Agreement), a kind of industry standard, which specifies not only the external dimensions of an optical transceiver but also electrical and optical characteristics, communication protocols with a host system for monitoring and control, arrangement of pin outs, and so on. Currently, CFP MSA has been established as a standard of the 100 Gbps optical transceiver and some commercial products have been released and delivered. The CFP optical transceiver brings a 100 Gbps transmission technology to the optical transmission systems by using some leading-edge techniques like WDM (Wavelength Division Multiplexing) of 4 wavelength channels and the 25 Gbps transmission per channel. In addition., Pulse Amplitude Modulation (PAM) system has been recently studied as a candidate of a new 100 Gbps transmission technology.